The invention concerns a process for the production of a filter.
Such a process is known from U.S. Pat. No. 5,139,624. In this process, the filter is produced by etching a p- or n-doped silicon blank. The silicon blank forms the underside of a vessel, filled with etching liquid, wherein an O-ring and a pressure clamp, which press the silicon blank against the vessel, are provided for the sealing. During the etching process, the silicon blank is connected to the anode of a current source, and a cathode is immersed in the etching liquid. The silicon blank thus forms a "working electrode," which is electrochemically perforated as a function of the doping, the current density, and the composition of the etching liquid.
DE 4202454 C1 describes a similar etching process, in which an n-doped silicon disk is additionally radiated with light for the purposeful production of minority charge carriers. The minority charge carriers move to the side of the silicon disk, which is wetted with etching liquid--preferably to the surface recesses, on which the electrical field strength is increased and the etching erosion is particularly strong. Holes are formed at these places and their formation can be influenced by the illumination.
U.S. Pat. No. 5,348,627 describes a semiconductor etching process, in which lenses or other optical devices, such as reflecting or partially absorbing masks, are also provided, through which the irradiation of the silicon blank can take place. The masks are used for the production of collimated irradiation, and the lenses, for the variation of the light intensity. The optical devices are therefore used for the purposeful "control" of the direction of the light rays, wherein it becomes possible for only certain "zones"--that is, for only certain partial areas of the silicon blank which are to be etched, to be irradiated, which involves a great technical outlay.
A similar etching process for the production of holes or pits in n-doped silicon is described in EP 0 296 348 A1, in which the substrate is also connected as an anode, is located on the underside of a vessel filled with a hydrofluoric acid-containing electrolyte, and is irradiated with light.
With the process of the state of the art, in which the blank to be etched is located on the underside of the etching vessel, a sealing of the chemically very aggressive etching liquid is difficult and involves an undesired great outlay and high costs.
In such arrangements, the case may also arise that on the boundary areas of the blank, the etching takes place at an angle to the surface, which is undesirable. In order to obtain a filter surface of a defined size, the boundary areas must be cut off after the etching treatment, wherein the production expense is further increased.
Another problem, which appears, in particular with the mass or serial production of filters, is the maintenance of identical production parameters--that is, the production of filters with identical characteristics (pore diameter, surface density of the pores, filter area, etc.). With the other processes, namely, the finished filter must be removed from the etching vessel. This requires an emptying and a subsequent refilling of the vessel, which is not desired with regard to the maintenance of constant process parameters.
Other filters for the separation of various substances in media, such as fluids, are, for example, membrane filters, network filters, deep-bed filters, or ultrafilters, which are made, for example, of mixtures of cellulose acetate and cellulose nitrate, cellulose polymers, woven nylon or metal threads, metallic silver, glass fibers, and microglass fibers.
The disadvantage with these filters is that they do not have an exactly defined hole structure and in particular, their pore diameters are not clearly defined. Liquid is kept in channel-like structures of the filter due to surface tension and when using a certain pressure, the liquid is expelled. In the so-called bubble test for the integrity test of filters, the corresponding formula is as follows: ##EQU1##
P: Bubble point pressure PA1 d: Diameter of the pores PA1 K: Shape correction factor PA1 e: Angle of contact between the liquid and the solid PA1 o: Surface tension
The pore diameter d goes into this experimentally obtained formula. From the formula, it is clear, however, that the influence of the other parameters leads, nevertheless, to undesirable substances or particles with dimensions that are larger than the approximation value d for the hole diameters passing through the filter.
Another disadvantage of the known filters is that they are not resistant chemically and are attacked, for example, by diverse acids, such as concentrated nitric acid, concentrated sulfuric acid, or concentrated hydrochloric acid, numerous solvents, such as, among others, methylene chloride, perchloroethylene, etc., and gases, such as ozone, which renders their use problematic or in some application cases, impossible.
Furthermore, many known filters are also not temperature-resistant. For example, traditional membrane filters made of polyvinyl dichloride can be autoclaved only up to ca. 130.degree. C. according to the manufacturer's data. Nylon network filters are resistant between -45.degree. C. and +115.degree. C. only when dry, but not during the filtration. Isopore membrane filters are temperature-resistant only up to ca. 140.degree. C.
Moreover, the costs of such filters rise considerably with the filter diameter in a direction transverse to the flow direction of the fluid.